If you do a big stimulus when the population is growing, you can expect, over time, to be able to spread debt payments over more people. The value of the debt may not change, but the per-capita debt burden will shrink.
But if you undertake a big stimulus when the population is stagnant, or even declining, then over time, the per-capita debt burden will rise … and if your society encourages long retirements, the debt burden per-worker will rise even faster.
That may change the cost-benefit calculation quite a bit when you’re considering a stimulus program. Not so much in the U.S., at least right now, because our population is growing. But this may suggest that however painful austerity has been for Europe, the austerians still did the right thing.